This magnitude 7.1 earthquake started deep underground, in a gash on the Atlantic seafloor, a little more than 650 miles off the coast of Liberia, in western Africa. It rushed eastward and upward, then did an about-face and boomeranged back along the upper section of the fault at incredible speeds‑so fast it caused the geological equivalent of a sonic boom.
The ferocity of shaking from an earthquake is usually focused in the direction the tremor is travelling. But a boomerang quake, or a “back-propagating rupture” in scientific terms, may spread the intense shaking across a wider zone. It remains uncertain how common boomerang earthquakes are—and how many travel at such great speeds. But the new study, published today in the journal Nature Geoscience, is a major step toward untangling the complex physics behind these events and understanding their potential hazards.
“Studies like this help us understand how past earthquakes ruptured, how future earthquakes may rupture, and how that relates to the potential impact for faults near populated areas,” says Kasey Aderhold, a seismologist with the Incorporated Research Institutions for Seismology, via email.
A kick in the ground
The latest boomerang was recorded near the mid-ocean ridge in the Atlantic, where the South American and African tectonic plates slowly inch apart. In spring 2016, scientists placed 39 seismometers near the ridge to collect the rumbles of distant quakes in an effort to visualise the base of the tectonic plate.
Several months later, the magnitude 7.1 quake rumbled by. It struck on a nearby fault in what’s known as the Romanche Fracture Zone, says Stephen Hicks, an earthquake seismologist at the Imperial College London and first author of the new study.
The fleet of seismometers faithfully recorded the shaking ground in a series of squiggles, including what seemed to be a pair of pulses. Intrigued, Hicks and his colleagues looked closer, identifying what seemed to be two steps of the quake. By examining the position of the epicentre and the energy released by each rumbling phase, the team connected the geologic dots: The quake initially headed eastward, but then turned back west. “This was a weird sort of configuration to see,” he says.
The team was still uncertain the earthquake actually boomeranged back and forth. So Hicks reached out to Ryo Okuwaki of the University of Tsukuba in Japan, who looked for the faint echoes of the event caught by other seismometers around the world. In just a few days, analysis of these distant traces provided an answer: The quake had likely boomeranged.
Further computer modelling suggested that the quake may have started deep underground rushing eastward until it neared the mid-ocean ridge. There, it turned back and raced through the upper section of the fault. This second leg of the tremor moved remarkably quickly, at so-called supershear speeds. The quake unzipped the surface at an estimated 11,000 miles per hour—fast enough to dart from New York to London in 18.5 minutes. This is so quick that the seismic waves pile up much like the Mach cone that forms from pressure waves as an airplane flies at supersonic speed. The concentrated cone of waves from a supershear quake can further amplify a quake’s destructive power.
A bevy of boomerangs
Understanding when and why these boomerang events happen is vital to grappling with the array of risks earthquakes present. Shaking from a quake can focus near one the end of the fault, in the direction the tremor is travelling, which is similar to the muting of the high pitch tones of a horn as a train rushes by. “Like the Doppler effect,” says seismologist Lingsen Meng of the University of California, Los Angeles, who was not part of the study team. While this focused shaking is usually thought to happen in one direction, a boomerang could focus shaking in two opposite zones. And if it were supershear, the rattling could be even worse.
But at least one big question remains: How often does this happen?
A boomerang quake at supershear speeds, as the team observed in the Atlantic, may be a fairly rare breed. “To the best of my knowledge, this is the first time it has been reported,” says geophysicist Yoshihiro Kaneko of GNS Science in New Zealand, who was not part of the study team.
But wider evidence of boomerang quakes is mounting. These back-tracking events have been studied in computer models as well as simulated in lab experiments. “The theory says that it’s there, but it’s quite difficult to see that [in the real world],” says geophysicist Louisa Brotherson, a PhD researcher at the University of Liverpool in the U.K., who simulates earthquakes in the lab.